Sulfur-oxidizing bacteria are gram-negative autotrophic bacteria and gain energy to grow by oxidizing sulfur substances to sulfuric acid. Taking advantage of this characteristic, application of sulfur-oxidizing bacteria to industrial fields typically includes bacterial leaching to recover copper by leaching it from copper sulfide ores (“Biseibutsu Shigenkougaku” (in Japanese) written and edited by Tadashi Chida (1996), Corona Publishing Co., Ltd. (Tokyo), pp. 74-83) and improvement of alkaline soil (JP Patent Publication (Kokai) No. 11-319795A (1999)). However, there is a problem that the ability of sulfur-oxidizing bacteria to oxidize sulfur is markedly inhibited at high chloride ion concentration because of influence of osmotic pressure and toxicity of chloride ion.